A crucial gap in the understanding of endothelial cell (EC) junction maintenance is the absence of a molecular mechanism to explain how angiopoietin-1 (Ang1) and its receptor, Tie2, stabilize the junctions. Loss of junction integrity i implicated in numerous diseases, including cancer, stroke, diabetes, rheumatoid arthritis, atherosclerosis, cardiac ischemia, and macular degeneration. In such pathologies, vascular endothelial growth factor (VEGF) has a potent disruptive effect on cell junctions and undermines vessel integrity. In contrast, angiopoietin-1 (Ang1) opposes the effect of VEGF and maintains junction integrity. Similar to Ang1, the formin protein mDia has a stabilizing effect on cell junctions, due to its maintenance of the cortical actin ring. We have obtained exciting preliminary results, which support the premise that mDia is an Ang1 effector. Based on these results, our proposal is expected to provide the first detailed account of the antagonistic effects of VEGF and Ang1 on cell junctions. Our central hypothesis is that VEGF and Ang1 regulate EC junctions by determining the spatial pattern of the activities of mDia and Syx (a RhoA-specific guanine exchange factor upstream of mDia) via membrane traffic. We will address this hypothesis by pursuing three specific aims: (1) determining how Ang1 recruits Syx to EC junctions; (2) determining the role of mDia trafficking in Ang1 and VEGF signaling; (3) determining how Syx and mDia regulate Ang1 signaling in a murine model of glioblastoma. To this end, we will use mouse and zebrafish loss-of-function models of several of the genes relevant to these pathways. The potential contribution of this study is significant because it will advance the field conceptually by integrating VEGF and Ang1 signaling into a coherent regulatory mechanism of vessel permeability. This will have a lasting effect on the general understanding of cell-cell junctions. A consequence of the poor knowledge of Ang1 regulation of cell junctions is the scarcity of drugs to target the Ang1 signaling pathway. This putative pathway, leading from Ang1/Tie2 to mDia and to the stabilization of endothelial cell junctions, implicates several proteins that have not been considered before as drug targets. The elucidation of this signaling pathway will provide, therefore, a solid foundation for the design of new therapies to prevent vessel leakage in pathological conditions. As a principal outcome, we will define key checkpoints that will allow us to selectively control VEGF vs. Ang1/Tie2 mediated angiogenesis. These features constitute the potential translational significance of our proposed research. The proposed research is innovative in that it will reveal a novel signaling pathway to explain how Ang1 stabilizes EC junctions, and will incorporate membrane traffic as a novel component of angiogenesis.